Typically, such stoppers consist of an elongate cylindrical refractory ceramic body of isostatically-pressed graphite/alumina having at the lower end a rounded or tapered profile (the stopper nose) suitable for engagement in the throat of a corresponding exit nozzle, and at the upper end some form of connecting means to fasten the stopper onto an external lifting mechanism by which the flow is controlled.
Operation of the stopper is simple in principle. A mechanical lifting system is used to vertically lift the stopper rod from a seating position on the nozzle to ease or restrict the volume of the molten metal flowing through the nozzle. However, in practice, such a stopper rod has to operate under harsh environmental conditions such as being submerged in the molten metal for long periods of time and must be able to withstand the high thermal shocks encountered in the pouring processes.
The cost of a stopper is essentially due to handwork and the quantity of refractory material. Attempts have already been made to reduce the cost by reducing the weight of the stopper. In such a case, care must be taken to avoid loss of strength. EP-A-625,391 discloses a fluted stopper, the outer surface of which being provided with a number of axial grooves. The exterior surface of the stopper has an undulating contour comprising alternate lobes and recesses to provide a fluted design which is said to retain the strength of a regular cylindrical stopper. However, the time needed to remove the stopper from its complex mould, largely overbalance the economy of refractory material.
It is therefore desirable to provide a stopper with a design allowing to use less refractory material without loss of strength while keeping the handwork as low as possible. The present invention aims to provide an improved stopper fulfilling these objectives.